The present invention relates to a printing ink-decorated packaging material of laminate type for aseptic packages according to the preamble to independent Claim 1. The present invention also relates to a method of producing the printing ink-decorated packaging material according to the preamble to independent Claim 7.
Within the packaging industry, use is often made of dimensionally stable, liquid-tight packages of a single use disposable nature for packing and transporting liquid foods under aseptic conditions so that the packed food, without the risk of bacteriological or other attack, may be stored for lengthy storage times (so-called extended shelf-life) in the unopened package with retained pristine quality and freshness. A large group of so-called aseptic single use disposable packages is produced from a packaging material of laminate type comprising a core layer of paper or paperboard and outer, liquid-tight coatings of plastic, normally polyethylene, on both sides of the core layer.
In order to impart to the aseptic package additional tightness properties, for example tightness properties vis-xc3xa1-vis light, oxygen gas etc., the packaging material is provided with at least one additional so-called barrier layer between the core layer and one of the two outer, liquid-tight plastic coatings which is intended to face inwards when the packaging material is reformed into packages.
Such a barrier layer may consist of a polymer, for example polyamide, although the barrier layer most generally consists of a foil of aluminium (Alifoil) which, apart from superior oxygen gas and light barrier properties, also possesses the advantage that it makes for mechanically strong and bacteriologically tight sealing joints by means of rapid inductive thermosealing technique, as a person skilled in the art will be aware.
Conventionally, aseptic packaging material of the type described above is produced in that a web of paper or paperboard is coated with outer, liquid-tight plastic coatings, normally polyethylene, which are extruded on both sides of the web. The plastic-coated web is thereafter provided with an aluminium foil which serves as oxygen gas and light barrier and which is brought together with and bonded to the outer plastic coating on one side of the web by means of an adhesive applied between the web and the aluminium foil. The web is then coated with one or more additional layers of plastic, normally polyethylene, which are extruded on the foil-clad side of the web for the formation of one of the two outer liquid-tight coatings of plastic described by way of introduction, i.e. that coating which is to face inwards when the packaging material is reformed into packages. The other of the two above-mentioned outer plastic coatings is normally provided with decorative artwork (hereinafter referred to as dxc3xa9cor) of printing ink, preferably water-based printing ink, which is applied on the outside of the plastic coating in the desired pattern by means of conventional printing techniques.
From the above-described packaging material, aseptic packages are produced in that both the packaging material and the relevant contents are sterilized prior to the filling operation, whereafter the filling phase is carried out under sterile (aseptic) filling atmosphere in order to avoid reinfection of the sterilized contents.
Nowadays, the production of packages most generally takes place using modern, high-speed packing and filling machines of the type which, either from a web or from prefabricated sheet blanks of the packaging material, form, fill and seal the packages.
From, for example a web, aseptic packages are produced in that the web for the purpose of sterilization, is led down into and through a bath of aqueous hydrogen peroxide solution, whereafter the web is dried and reformed into a tube by both longitudinal edges of the web being united with one another in an overlap joint seal. The tube is filled with the relevant sterilized contents, for example liquid food, and is divided into closed, filled packages by repeated transverse seals below the level of the contents in the tube. The packages are separated from one another by incisions or cuts in the transverse sealing zones and are given the desired geometric, normally parallelepipedic configuration, by a further forming and sealing operation for the formation of the finished, aseptic packages.
Printing inks for the described application are most often water-based and therefore require that the surface on which they are to be applied (in this case the one outer plastic coating of the packaging material) is sufficiently hydrophilic to achieve good adhesion between the printing ink and the printing surface.
A surface of polyethylene, which is used as the dxc3xa9cor-carrying plastic surface in the above described prior art packaging material is, however, hydrophobic (water repellent) and must therefore be modified in order to make for the desired good adhesion. For the purpose of promoting adhesion, the packaging material is therefore subjected to a surface treatment modifying its polarity which is normally based on the concept that the plastic surface in question is treated with electric corona discharges (so-called corona treating), whereby the plastic surface is given the desired hydrophilic character prior to the application of the water-based printing ink dxc3xa9cor.
One serious problem inherent in the prior art printing ink decorated packaging material is that at least certain of the printing ink components present in the water-based printing ink tend to interact with hydrogen peroxide when the printing ink-decorated packaging material is led down into and through the hydrogen peroxide bath, as described above. Such an interaction not only leads to the printing ink dxc3xa9cor of the packaging material being destroyed (xe2x80x9cfadingxe2x80x9d), but also results in excessively high consumption of hydrogen peroxide because of the undesired interaction.
One object of the present invention is therefore to obviate the above-outlined drawback in the prior art technique.
A further object of the present invention is to realise a printing ink-decorated packaging material for aseptic packages which is well-adapted for sterilization by means of aqueous hydrogen peroxide solution, without the risk of interaction between the printing ink and the hydrogen peroxide leading to a deterioration in quality, and without excessive consumption of hydrogen peroxide.
Yet a further object of the present invention is to realise a printing ink-decorated packaging material for aseptic packages which is susceptible to sterilization using aqueous hydrogen peroxide solution with retained pristine printing ink quality and with but minimum consumption of hydrogen peroxide as compared with the prior art printing ink-decorated packaging material.
Still a further object of the present invention is to produce a printing ink-decorated packaging material for aseptic packages, the packaging material displaying, in addition to good printing properties, particularly good liquid tightness properties.
Solution
These and other objects are attained according to the present invention in that a packaging material of the type described by way of introduction has been given the characterizing features as set forth in appended Claim 1. Advantageous embodiments of the packaging material according to the present invention have further been given the characterizing features as set forth in the appended subclaims.
One explanation for the above described interaction problem, and the intimately related excessive consumption of hydrogen peroxide, may be that the surface treatment carried out for the purpose of promoting adhesion by means of electric corona discharges (corona treating) is so powerful that it not only generates the intended surface modification of the outer plastic coating of the packaging material (printing surface), but is even so powerful that the outer plastic coating is xe2x80x9cbroken upxe2x80x9d because of the electric discharges. Such xe2x80x9cbroken upxe2x80x9d regions of the plastic coating have such surface polarity (hydrophilic) that they tend to excessively absorb and retain hydrogen peroxide when the packaging material is brought into contact with the aqueous hydrogen peroxide solution in connection with the reforming of the packaging material into aseptic packages, as described above.
This problem is solved elegantly and effectively according to the present invention in that the printing ink-decorated packaging material is provided with a protective layer of a water-based dispersion of polyethylene or acid-modified polyethylene applied on the dxc3xa9cor-carrying outer plastic coating of the packaging material.
A protective layer of water-based polyethylene or acid-modified polyethylene gives the printing ink-decorated outer plastic coating an effective protection against printing ink/hydrogen peroxide interaction, at the same time as it effectively protects such xe2x80x9cbroken upxe2x80x9d regions of the corona-treated plastic coating against contact with aqueous hydrogen peroxide solution in connection with the sterilization of the packaging material, whereby consumption of hydrogen peroxide is drastically reduced.
Examples of usable acid-modified polyethylene dispersions for the protective layer in the packaging material according to the present invention are ethyl acrylic acid copolymer (EAA) and ethyl methacrylic acid copolymer (EMA), preferably ethyl acrylic acid copolymer (EAA).
In another aspect of the present invention, a method is realised of producing the printing ink-decorated packaging material according to the present invention, which is characterized in that the outer plastic coating serving as the dxc3xa9cor carrier is coated with an aqueous dispersion of polyethylene or acid-modified polyethylene by so-called dispersion coating.
Dispersion coating, which is a known coating method, may be integrated with the printing phase and thus makes for a rational production and distribution of printing ink-decorated packaging material, even if the printing ink-decorated packaging material is to be produced in very small series for extremely limited markets.
More precisely, the packaging material production may be carried out in two main stages at separate worksites of which the first or central stage is based on producing a prefabricated material, i.e. a non-decorated but otherwise complete packaging material, in normal industrial production volumes in a central, separate production plant, and from the central plant shipping out such prefabricated material to smaller, local production plants where the non-decorated semi-manufactured packaging material is provided with the desired local dxc3xa9cor and coated with its protective layer by dispersion coating.